Structural basis for the versatile interactions of Smad7 with regulator WW domains in TGF-β Pathways.

Transforming growth factor (TGF)-β and BMP signaling is mediated by Smads 1-5 (R-Smads and Co-Smads) and inhibited by Smad7, a major hub of regulation of TGF-β and BMP receptors by negative feedback and antagonistic signals. The transcription coactivator YAP and the E3 ubiquitin ligases Smurf1/2 and Nedd4L target R-Smads for activation or degradation, respectively. Pairs of WW domain in these regulators bind PY motifs and adjacent CDK/MAPK and GSK3 phosphorylation sites in R-Smads in a selective and regulated manner.

A Smad action turnover switch operated by WW domain readers of a phosphoserine code.

When directed to the nucleus by TGF-β or BMP signals, Smad proteins undergo cyclin-dependent kinase 8/9 (CDK8/9) and glycogen synthase kinase-3 (GSK3) phosphorylations that mediate the binding of YAP and Pin1 for transcriptional action, and of ubiquitin ligases Smurf1 and Nedd4L for Smad destruction. Here we demonstrate that there is an order of events-Smad activation first and destruction later-and that it is controlled by a switch in the recognition of Smad phosphoserines by WW domains in their binding partners. In the BMP pathway, Smad1 phosphorylation by CDK8/9 creates binding sites for the WW domains of YAP, and subsequent phosphorylation by GSK3 switches off YAP binding and adds binding sites for Smurf1 WW domains.

Ubiquitin ligase Nedd4L targets activated Smad2/3 to limit TGF-beta signaling.

TGF-beta induces phosphorylation of the transcription factors Smad2 and Smad3 at the C terminus as well as at an interdomain linker region. TGF-beta-induced linker phosphorylation marks the activated Smad proteins for proteasome-mediated destruction. Here, we identify Nedd4L as the ubiquitin ligase responsible for this step.